When a portable cellular telephone is used within a motor vehicle, the user frequently experiences a decrease in performance of the telephone reception and transmission due to the fact that the built-in antenna on the telephone is partially shielded by the metal body of the vehicle. In order to eliminate or minimize this signal degradation problem, it is desirable to connect the antenna on the telephone to a remote externally located antenna which may be mounted on a window or on the vehicle body by a magnetic base. Some portable cellular telephones have a female jack for receiving a plug on a coaxial cable connected to an external antenna. However, many portable telephones do not have such a jack and incorporate retractable, whip-like or flexible wire antennas which usually have a length of about 4 to 4.5 inches and a diameter between 0.075 and 0.095 inch. Such antennas are retractable into the housing of the telephone and are extended when it is desired to use the telephone.
Other types of portable cellular telephones or transceivers have a non-retractable flexible antenna with a helically wound conductor covered by a rubber or plastic tube and commonly known as a "rubber duck" antenna. An R-F coupling device for such an antenna is disclosed in U.S. Pat. No. 4,220,955. This device consists of an inner non-metallic tube or sleeve which is adapted to telescope over the flexible antenna on the transceiver. The inner tube is surrounded by a helically wound conductor or coil, and an electrically conductive cylindrical casing with end caps surrounds the helical coil and insulating sleeve.